Implement std::pmr::unsynchronized_pool_resource
	* config/abi/pre/gnu.ver: Add new symbols.
	* include/std/memory_resource (std::pmr::__pool_resource): New class.
	(std::pmr::unsynchronized_pool_resource): New class.
	* src/c++17/Makefile.am: Add -fimplicit-templates to flags for
	memory_resource.cc
	* src/c++17/Makefile.in: Regenerate.
	* src/c++17/memory_resource.cc (bitset, chunk, big_block): New
	internal classes.
	(__pool_resource::_Pool): Define new class.
	(munge_options, pool_index, select_num_pools): New internal functions.
	(__pool_resource::__pool_resource, __pool_resource::~__pool_resource)
	(__pool_resource::allocate, __pool_resource::deallocate)
	(__pool_resource::_M_alloc_pools): Define member functions.
	(unsynchronized_pool_resource::unsynchronized_pool_resource)
	(unsynchronized_pool_resource::~unsynchronized_pool_resource)
	(unsynchronized_pool_resource::release)
	(unsynchronized_pool_resource::_M_find_pool)
	(unsynchronized_pool_resource::do_allocate)
	(unsynchronized_pool_resource::do_deallocate): Define member
	functions.
	* testsuite/20_util/unsynchronized_pool_resource/allocate.cc: New
	test.
	* testsuite/20_util/unsynchronized_pool_resource/is_equal.cc: New
	test.
	* testsuite/20_util/unsynchronized_pool_resource/options.cc: New
	test.
	* testsuite/20_util/unsynchronized_pool_resource/release.cc: New
	test.

From-SVN: r265853